1. Field of the Invention
The present invention relates to rollers and sleeves for rollers which are used in printing processes. The invention more particularly relates to coatings and compositions for coatings which may be used on rollers and sleeves on apparatus which prints ink onto receptor materials. The coatings may display a range of properties including, but not limited to, thermal stability, durable hardness, grindability, and flexibility.
2. Background of the Art
The process of providing images on substrates by printing actually involves a wide variety of different types of processes. The technology has advanced substantially from the type-set and carved substrate lithographic printing of the 16.sup.th through 19.sup.th century to include such varied processes as gravure printing, relief printing, flexographic printing, lithographic printing, photolithographic printing, offset printing, and numerous other processes which are included within the broad terminology of printing. Even processes that do not use imaged substrates with differential images (in relief or in ink affinity) to transfer images are mischaracterized as printing processes. Such misnamed processes would include laser jet printing, bubble jet printing, and even some forms of electrostatic printing (where there is no transfer of a toned electrostatically developed image).
The traditional printing process involves the formation of an image on a substrate, the image having the capability of accepting ink thereon in an imagewise distribution which will allow the ink to be transferred in the form of an image to a receptor surface. The capability of accepting ink in a differential form may be effected by a relief image on a surface, a differentially hydrophilic/hydrophobic image area, or the like.
In the actual printing process, it is common to use rollers, usually cylindrical elements, in many different phases of the process. The rollers may be used, for example, to support a printing plate or receptor material, transfer a printing plate or receptor material, carry ink, transfer ink, press the printing plate or inking pad, press the printing plate into contact with a receptor material, remove excess materials (e.g., inks, fountain solutions or coatings), develop the image, or dry the printed image. Rollers can directly affect the quality of the printed image because of their potential direct involvement in the position of materials, contact with the printed image and receptor materials, transfer of ink to the printing surface, and forces directly involved in the printing step. Defects in rollers, such as surface irregularities, concavity and convexity can cause build-up of materials on the surface of the roller. Such defects can easily be seen to cause reduction in printing quality where the rollers transfer erroneous amounts of ink and/or differentially apply pressure to the printing plate when the plate is transferring ink during the printing process. It is therefore necessary that the rollers provide a uniform and durable surface, and it is desirable that these types of surfaces be provided at minimal cost.
The surfaces of rollers can provide the desired surface characteristics by either mechanical milling of the roller surface, coating of the roller surface, or application of a sleeve to the roller surface. Sleeves, as the name implies, are placed over the rollers in a tight fit and may even be shrunk over the roller to provide an exceptionally tight fit. The use of sleeves allows for the use of roller substrates of various materials and allows the rollers to be adapted to different printing processes by altering the composition and structure of the sleeve. A sleeve is different from a coating. Coatings are usually applied as liquids or laminable film to a roller surface. As the applied surfaces (coatings and sleeves) wear down over time, the sleeves offer one type of advantage in their being able to be replaced without chemical treatment or grinding of the roller.
Many conventional rollers used in the flexographic printing industry are prepared from fiber reinforced (e.g., glass fiber) polymers, especially polyester resin. The rollers can be manufactured by building up layers of fiberglass and resin on a support (e.g., of the shape and dimensions desired), and then solidifying the reinforced resin mix. The thickness may be built up by additional layering if desired, and a top coat of polyester resin may be applied to the surface. This top coat may be machined or ground to the diameter desired. The top coat may be applied by spraying and curing.
DE 195529809 A describes a high heat conductivity coating for printing rollers which comprises an elastomeric resin with up to 50% by weight of filler. The resin material may be heat-vulcanized rubber, nitrile-butadiene rubber, phenolic, epoxy, polyurethane, polyester, silicone, acrylic or acrylate resin systems, and the filler may be carbon black and/or silanized silica to improve the heat conductivity. The coating thickness is usually below 100 micrometers, especially between 5 and 40 micrometers, the applied coating is usually thermally post-treated. The coating compositions may be applied from a solution or dispersion, especially by spraying.
EPO 566 418 A describes an electrically conductive or semiconductive polymeric material comprising a metal salt dissolved in a polymer. The metal salt is complexed with the polymer to provide a material with a resistivity of 10.sup.12 to 10.sup.5 ohms/cm. The material comprises less than 5% by weight of the metal salt, which is usually a transition metal halide, although copper lactate, copper larate, iron phosphate and iron oxalate are described. The metal salts are described as having molecular sizes of 1-10 Angstroms. The polymers in which the metal salts are included are elastomeric polymers and rubbers such as nitrites, natural rubbers, neoprene, fluorocarbons and silicones. The coating compositions are disclosed as useful on rollers, including pick-up rollers on paper printers.
U.S. Pat. No. 5,445,886 describes a printing roller consisting of a metal core with a surface layer with a lattice-fabricated groove structure on the surface of the roller. The surface of the roller is coated with elastomeric materials of polyurethane resin, silicon rubber, acrylates, epoxy resin, phenolic resin, nitrile rubber, fluororubber and nitrile rubber.
Soviet Union Patent Application SU 3473782 describes a copper-containing dimer isocyanate oligomer of the formula Cu.sub.3 (C.sub.6 H.sub.3 CH.sub.3 (NCO).sub.2).sub.6 Cl.sub.6, obtained by the reaction of cupric chloride with 2,4-toluene diisocyanate in acetone or tetrahydrofuran. A multi-nuclear complex is formed which provides a novel antistatic polyurethane for use in the manufacture of antistatic coatings, camera winding rollers and print rollers.